My speedo sometimes works, sometimes it doesn't. It may be ambient temp related, anyway, what it does:

When i am cranking the engine over to start the car, the speedo needle bounces around pretty good, sometimes to 160 MPH, usually only up 120 though, 0-high, 0-high, bouncing.

I then drive the car and speedo shows 0. after a while it will usually, but not all the time, start working.

If the speedo is working, I can stop at a light or something and then go, it will till work.

If it is working and shut the car off, even for few seconds, it often won't work on the restart,and it may, or may not start working again.

Higehr temps seem to make it work more often.

Does this sound like a bad connection somewhere? Or maybe flaky sending unit or transducer? The bouncing on start is peculiar, but it tells me the gauge itself works..

Thanks,
Doug

 

Sounds like the speed sensor is on its way out.

Your car has the sensor in the diff right(electronic sensor vs mechanical like found in older cars)

 

Hi Doug

At the Rear of the Binnacle that holds the Speedo and the Tacho, is a very delicate Printed Circuit, which may have some Areas of Corrosion that need to be Cleaned with Lemon Juice and then the Rubber on the end of a Pencil

Its quite a delicate pains taking job that could take you a couple of Hours just to clean it. as you have to be Careful not to Break the Printed Circuit

Your problem is almost Certainly down to a 'dodgy earth', so you can also fit an Auxiliary Earth while you've got the Binnacle out

In the event that none of this works then it could be the 'Little Black Box Interface', that sits near the Top of the Internal Wheel Arch (Fender) in the Boot Trunk. The Interface Splits the Signal into Analogue and Digital

The Digital Does The Trip Computer and the Analog does the Speedo (Although You might no need to know that) if its down to Corrosion on the Printed Circuit- Lol!

Removing the Actual Binnacle is easy (even the GF can do it) or should that be 'Did it!'

Don't remove the Plug from the Diff, not unless you absolutely have to, or you could be left with an even bigger problem than the one you Started With. as that Sensor has to go back with just the right amount of Shims and is a Total PIA to get to

Since you are Taking the Binnacle out, then why not upgrade the Bulbs in the Binnacle for Better Lighting to the Speedo and Tacho, as there would be no better time than now to do that job

Brighter Binnacle Lights for the Speedo and the Tacho

Fixing the Speedo on a Pre-Facelift V12 XJS

 

Good info, thanks!

Couple questions:

Do you have better spec/callout/part number on those instrument bulbs? All I saw referenced was 5w, Classic, and e-bay.

And you mention adding an aux ground/earth. I can't see the full circuit from the pics, but I assume one of those copper strips the multi plug connects to is the ground. Are you referring to adding a totally separate ground wire attached somewhere on the instrument cluster/binnacle? Or finding the ground wire on the multi plug and tapping into it with another wire directly to good ground? I would assume one of the speedo screws could be used as a ground wire attaching point.

Thanks,
Doug
QUOTE=orangeblossom;2698630]Hi Doug

At the Rear of the Binnacle that holds the Speedo and the Tacho, is a very delicate Printed Circuit, which may have some Areas of Corrosion that need to be Cleaned with Lemon Juice and then the Rubber on the end of a Pencil

Its quite a delicate pains taking job that could take you a couple of Hours just to clean it. as you have to be Careful not to Break the Printed Circuit

Your problem is almost Certainly down to a 'dodgy earth', so you can also fit an Auxiliary Earth while you've got the Binnacle out

In the event that none of this works then it could be the 'Little Black Box Interface', that sits near the Top of the Internal Wheel Arch (Fender) in the Boot Trunk. The Interface Splits the Signal into Analogue and Digital

The Digital Does The Trip Computer and the Analog does the Speedo (Although You might no need to know that) if its down to Corrosion on the Printed Circuit- Lol!

Removing the Actual Binnacle is easy (even the GF can do it) or should that be 'Did it!'

Don't remove the Plug from the Diff, not unless you absolutely have to, or you could be left with an even bigger problem than the one you Started With. as that Sensor has to go back with just the right amount of Shims and is a Total PIA to get to

Since you are Taking the Binnacle out, then why not upgrade the Bulbs in the Binnacle for Better Lighting to the Speedo and Tacho, as there would be no better time than now to do that job

Brighter Binnacle Lights for the Speedo and the Tacho

Fixing the Speedo on a Pre-Facelift V12 XJS[/QUOTE]

 

Hi Doug

Those Bulbs are nothing special. just 5 Watt Bayonet Classic Auto Bulbs from ebay or your local Auto Shop and they do make a Heck of a difference and you will be as pleasantly very surprised as I was!

I would avoid LED's but that is up to you, as they can be a bit problematical

The Earth is a Separate Earth just like you mentioned (although you may not really need it, as its a bit of a Back up Plan)

The Tacho should Bounce when you Crank the Engine about 25mm as that is an indication that the Crank Position Sensor is working ok

Hope This Helps

Alex

 

Quote:

When i am cranking the engine over to start the car, the speedo needle bounces around pretty good, sometimes to 160 MPH, usually only up 120 though, 0-high, 0-high, bouncing.

Classic symptom of voltage drop. Voltage supply is low, or poor ground, as mentioned. Cleaning the printed circuit, as mentioned, is something that every old XJS needs. Plus the extra ground wire. They just do. If you still get the bounce then voltage supply further upstream must be checked.

Quote:

Your car has the sensor in the diff right(electronic sensor vs mechanical like found in older cars)

Maybe, maybe not.

The diff mounted speed sensor came along at VIN 144xxx or 142xxx....something like that. This was later is the (USA) 1988 model year. Early '88 cars still the the speedometer transducer mounted on the transmission, LH side. The diff mounted speed sensor uses the little speedometer interface box. The transducer system did not use a speedometer interface....although there will be a fuel interface box for the trip computer

Quote:

The Earth is a Separate Earth just like you mentioned (although you may not really need it, as its a bit of a Back up Plan)

100% worth doing !

Quote:

The Tacho should Bounce when you Crank the Engine about 25mm as that is an indication that the Crank Position Sensor is working ok

On a Marelli ignition car. A Lucas ignition car, which Doug's likely is, wouldn't have the CPS.

Hate to pick nits here. Just trying to save Doug the effort of looking for things that his car may not have

Cheers
DD

 

The tach doesn't work until the motor has been running for several seconds, no bounce on cranking on it. You are probably right, bad ground, and maybe feeding back thru the speedo if it is bouncing on cranking.

Good info!

Doug

 

My car seems to be a hybrid kluge of early 1988 and later 1988 stuff. It is an H&E conv conversion. #146775. Tag date on the door jamb says 11/87 (maybe 12/87, I am working from memory) It does have a later ECU and later wiring based on the brake light circuit, but some things seem to be from an earlier car based on some relay locations under the hood, and I don't think someone changed them. It also has the little black speedo transducer box above the LH wheel well. That transducer wiring is cheesy as can be, little tiny wires going into some shielded cable that looks like residential CAT 5 phone cable.

Doug

 

Another possible instrument cluster ground related situation??

I did put LED bulbs in the rear only, of my turn signals.

The dash indicator lights will flash once, usually, sometimes not at all.

Looking at the later 1988 wiring diagram, I see no electronic gizmo that would cause them not to operate as they should, like any old normal car would with a couple LED lights. I know if you go all LED you need to replace the thermal flasher with an electronic one. I do not "think" there is anything special in the Jag flasher to cause this issue, but maybe there is, as I did read that failure of turn signal indicator to function, is a warning that one of your turn signal bulbs is out.

A simple rewire would fix this, but i prefer not to go that route as rewiring anything under dash related, is hard on the body once you are over about 45 years old.

Is there something special about the jag flasher that causes this? is there a replacement for it that won't cause the indicator lights to not work?

And no, I am not getting rid of the rear LEDS, I like to be seen.

Thanks,
Doug

 

Quote:

The dash indicator lights will flash once, usually, sometimes not at all.

But the exterior signal lamps are working normally?

I had this symptom once and replacing the flasher solved the problem. Don't ask me why.

Quote:

Looking at the later 1988 wiring diagram, I see no electronic gizmo that would cause them not to operate as they should, like any old normal car would with a couple LED lights. I know if you go all LED you need to replace the thermal flasher with an electronic one. I do not "think" there is anything special in the Jag flasher to cause this issue, but maybe there is, as I did read that failure of turn signal indicator to function, is a warning that one of your turn signal bulbs is out.

What happens if you switch your LED bulbs to incandescent?

Cheers
DD

 

5 watt push fits, Alex, not bayonets. Like this

 

The flashers will not work correctly with LEDs, or even two of the four. PTJS1 has done much work on this matter so worth reading his thread. The flashers work incorrectly when one of the four normal bulbs fails, this is a warning system and is in the handbook.